In ICs, various components such as transistors, resistors, inductors, capacitors and varactors are configured to achieve the desired function. Generally, varactors are employed in analog applications. Varactors are essentially capacitors where the capacitance value varies with the voltage applied. One common type of varactors is a junction varactor.
FIG. 1 shows a conventional configuration 100 having a structure of a capacitor 110. The capacitor includes a first electrode 112 and a second electrode 118 (cathode and anode). A first terminal 160 is coupled to the cathode and a second terminal 170 is coupled to the anode. The anode and cathode are separated by a dielectric region 114. The width of the dielectric region determines the capacitance of the varactor. In general, wider the dielectric region, lower the capacitance. Additionally, larger the electrode area, higher the capacitance. The anode and cathode of the varactor can be formed at a p-on-n or n-on-p junction, with the depletion region at the junction serving as a dielectric region. Since the width of the depletion region can be modulated with the applied bias across the junction, a voltage dependent variable capacitor (varactor) results.
FIGS. 2a-b show cross-section views of a conventional n-on-p (NP) junction varactor 100. The junction varactor 110 is formed on a substrate 205. The substrate includes a p-well 218. Shallow trench isolations (STIs) 280 define first 162 and second 172a-b regions. A heavily doped n-type region is provided in the first region, creating an NP junction 214 with the p-well. The n-doped region serves as the anode while the p-well serves as the cathode. Contact to the p-well is achieved through heavily doped p-type regions at the surface of the second region. The input and bias voltages are applied to the cathode 160 and anode 170 terminals respectively.
A depletion region, indicated by dotted lines 216a-b, occurs at the NP junction. The capacitance of the varactor corresponds to the width of the depletion region, which can be varied by adjusting the voltages at the terminals. When the NP junction is at zero bias (i.e., Vin=Vbias), the depletion width is at its minimum, as shown in FIG. 2a, corresponding to a maximum capacitance (Cmax). The width of the depletion region widens as the reverse bias across the junction is increased. Widening the depletion region decreases the capacitance of the varactor. When the maximum reverse voltage is applied, the depletion width is at its widest, as shown in FIG. 2b, corresponding to a minimum capacitance (Cmin).
An important factor is the tuning range of the varactor, which corresponds to the capacitance range in which the varactor operates and is defined by the ratio Cmax/Cmin. Generally, it is desirable for a varactor to have a large tuning range to provide better functionality. Although junction varactors have better linearity compared to MOS-type varactors, they suffer from a smaller tuning range.
From the foregoing discussion, it is desirable to provide a junction varactor with a large tuning range.